1. Field of the Invention
The present invention is directed to method and apparatus for forming bias laid, non-woven fabrics wherein, preferably, the yarns in at feast two of the paers of fabric are psid at an angle of from 30.degree. to 150.degree. to the long axis of the fabric. In such fabrics, the yarns in the various layers are neither knitted, nor woven, but are held together by stitching through the layers, or by other external means, such as adhesive bonding.
2. The Prior Art
The history of fabric formation is a long one. Most fabrics are made by the now traditional processes of knitting, weaving, etc., and sophisticated machinery has been developed for automatically manufacturing fabrics in accordance with these techniques.
For many modern usages, particularly in areas where structural strength and integrity are required, fabrics manufactured by the older techniques cannot be used. Such uses include structural parts for high speed airplanes where the fabric is to be impregnated with a curable resin system.
In the modern usages referred to, the traditional knitted or woven fabrics do not provide sufficient strength, even when impregnated with a curable resin system, following cure, to provide the necessary uniformity and strength. Accordingly, non-woven fabrics have been developed for such utilization.
The non-woven fabrics which have been developed for these structural uses involve a series of layers which are laid down, generally in a continuously formed fabric, and with at least the final width of the fabric during formation, the layers ultimately being held together by stitching through the layers, knitting with a loose stitch through the layers, or adhesively bonding threads of the layers at crossing points. The composition of the stitching material or of the adhesive material is not of critical importance, so long as the material has sufficient strength to hold the various layers together up to the time of resin impregnation, since the final strength of the part formed and the holding of the various yarns of the fabric in their proper position is accomplished by the cured resin.
The most desirable of the non-woven fabrics for structural purposes has been found to be those with at least two layers, the yarns of which are at an angle of approximately 45.degree. to the long axis of the fabric direction, the two layers lying at 90.degree. to each other. There can be more than two layers of yarns, depending upon the end use to which the fabric is to be put and either the first two layers, or any successive layers, can be placed at angles varying from 30.degree. to 150.degree. to the long axis of the fabric. If desired, a series of warp threads, lying parallel to the long axis of the fabric, a series of weft threads, lying at approximately 90.degree. to the long axis of the fabric, or both, can be included. Once all of the fabric layers have been placed, the fabric is held together for storage, shipment, and ultimate impregnation, by one of the referenced methods, i.e., stitching, loose weave knitting, or adhesive bonding.
Among patents showing the formation of similar, types of fabric are Eaton, U.S. Pat. No. 3,607,565; Smith, U.S. Pat. No. 3,765,893; and Campman et al, U.S. Pat. No. 4,325,999.
The Campman et al patent particularly describes a number of methods for forming bias laid, non-woven fabrics, as generally referred to in the present patent application. However, as will be observed from a review of Campman et al, successive courses of each set of yarns there are laid in a pattern such that each course is angled at 90.degree. to the previous course. For purposes of this invention, a course is defined as the plurality of yarns laid together in traversing the distance from one side of the fabric being formed to the opposite side: when the plurality of yarns reverses directions, and returns from the second side to the first side, that is a second course.
In Campman et al, prior to the reversal of direction of the yarns, so as to lay a second course, the yarns are wrapped around a series of pins, the number of pins corresponding to the number of yarns being laid. When the plurality of yarns is returned to the first side of the forming fabric, the yarns are wrapped about a set of pins formed on the conveyor on the first side, and, again, direction reversed by 90.degree. so as to be returned to the second side for a fourth course. Campman et al do show one embodiment in which the courses of yarns formed by a single set of moving yarns are parallel to each other. That is, essentially, shown in FIG. 10 of the Campman et al patent, and the portion of the disclosure relating to that figure. However, a relatively complex mechanism is necessary to accomplish this parallelism between courses, the complex mechanism including two sets of pins on each side of the fabric being formed to allow the second, or return course, to be parallel to the first. None of the other automatic types of bias fabric formation machinery known to applicant provide even a mechanism of this complexity for forming parallel courses.
The inability to provide parallel courses results, in many instances, in a diminution of strength of the structural member being formed from these bias laid, non-woven fabrics. Further, because there is a waste of yarn due to the 90.degree. return angle, which causes the second course to partially overlie the first course, the expense of the bias laid, non-woven fabric is greater than it would be if parallel courses were possible.
A method and apparatus which would provide for the yarns in successive courses to be laid parallel to those in previous courses, without the complicated mechanisms of the prior art, would be extremely valuable. Similarly, method and apparatus which would allow for the use, but spacing, of a number of yarns greater than the number of pins formed on the traveling conveyors would provide for greater flexibility in the formation of bias laid, non-woven fabrics, and the production, with relative ease, of fabrics tailored to particular structural uses, as dictated by the needs of those uses. Still further, some overlapping of courses can be provided for, but that overlapping can be controlled, again for the needs of the structural item to be formed, and not as dictated by the limitations of the method and apparatus for forming the bias laid, non-woven fabric.